1. Field of Invention
The present invention relates to systems for determining the chemical composition of gaseous mixtures. In particular, the present invention is directed to systems and processes for determining the presence and composition of oxygen, nitrogen and hydrocarbons, among other gaseous constituents, in gaseous mixtures. In specific embodiments, the invention allows for the use of optical spectroscopy to analyze the gaseous compositions without exposing the interiors of liquid receptacles to the outside or sources of ignition.
2. Description of Related Art
There are many environments where it is important to measure with great accuracy the composition of gaseous materials in real-time. These measurement environments can include measuring to determine the presence of harmful gases, the determination of intrinsic properties, or the monitoring of percentages of specific compositions. Such environments include a natural gas pipeline where monitoring of species can determine heating values or in monitoring of respiration or anesthesiological values in the delivery of medical treatments. The ability to measure such quantities in real-time would be of great interest to many fields of endeavor.
One such issue of determination of compositions of gaseous materials occurs with fuel tanks. In the area of aircraft safety, there exists a need to reduce or eliminate the explosion hazard posed by the mixture of fuel vapors and oxygen contained in the space above the liquid (ullage) of a fuel tank. In a conventional aircraft fuel tank, the ullage will contain a mixture of jet fuel vapor and air. As the fuel is used up, more air from the outside is drawn in to fill the ullage created by the spent liquid fuel. In order for a fire or explosion hazard to exist, there needs to be three things: fuel, oxygen, and energy to ignite the mixture.
Since the fuel cannot be removed, and ignition sources (such as static electricity, lightning, or even a terrorist strike) may exist, the oxygen needs to be reduced or removed. The oxygen is removed by displacing it with nitrogen enriched air that is produced from an on-board inert gas generation system (OBIGGS) that feeds nitrogen-enriched air into the fuel tank to eventually displace the oxygen. Such systems are expensive to operate continuously (from a fuel economy standpoint), and it also desirable to know when the fuel tank has been rendered inert to a safe level (typically less than 9% by volume). Thus, a way to measure the relative or absolute oxygen and/or nitrogen concentrations in the ullage of fuel tank is needed.
Unique features of such a problem include that the sensor system cannot introduce any intrinsic fire or explosion hazard such as would be the case with electronic wire-based sensors and the sensor system needs to provide a measurement of the oxygen concentration over a wide temperature (−40 C to 71 C) and pressure (0.2 atm to 1.0 atm) range corresponding to the conditions inside an airplane fuel tank over the flight envelope. The sensor needs to be reliable and robust and needs to be compact and lightweight in order to be fitted to an airplane's fuel system. The sensor should preferably also measure nitrogen and fuel vapor concentrations for a secondary check of the OBIGGS system performance and the system needs to work reliably and unattended with no maintenance for up to 1 year (time between fuel tank service checks). The system should also preferably be low-cost for deployment on the entire fleet of commercial aircraft. Specifically, the sensor needs to be able to measure the absolute concentration of O2 at the following condition which represents the lowest absolute concentration of molecular oxygen in the flight envelope. The sensor needs to have a 1% measurement precision and accuracy in measuring 5% O2 (by volume) at a temperature of −1 C and a pressure of 0.2 atm, with the balance gases being nitrogen and fuel vapor.
Prior art systems have relied on electrochemical oxygen sensors or paramagnetism oxygen sensors. A newer technique, that uses the effect of oxygen fluorescence quenching on a dye that is fiber optically illuminated by a blue LED, has been proposed. Other techniques have been proposed to use diode laser absorption spectroscopy near 760 nm to measure oxygen. However, the first two techniques above require exposing the ullage gases to a sensor that is connected to a wire which can potentially be a source of sparks or electrostatic ignition. Furthermore, the electrochemical sensor typically uses a heated element that reacts and consumes the oxygen that it is sensing, producing a fire hazard. The fiber optic fluorescence quenching sensors that utilize the intensity of an oxygen sensitive dye that is illuminated with a blue LED have also been proposed but this sensor is prone to large drifts when exposed to temperature changes. The drift of the fluorescence quenching type sensor is so great that it renders the technique ineffective for the reliable measurement of oxygen concentration in the ullage of a fuel tank. The diode laser absorption technique is problematic because it requires a diode laser to tune and lock onto specific oxygen absorption lines, the technique requires windows for optical access which can get dirty from fuel deposits. None of these techniques can measure both nitrogen and oxygen concentration to provide an accurate mixing ratio without having to assume that the balance is nitrogen. Also, none of these other techniques are able to provide fuel vapor concentration.
As such, there is a need for a system that does not introduce any intrinsic fire or explosion hazard, provides measurements of gaseous concentrations over wide temperature and pressure ranges and which is reliable and robust. There is also a need for a system that is compact and lightweight and be low-cost for deployment in many detection environments.